Nuclear reactors require measurement of the neutron fluence or neutron dose (i.e. the number of impinging neutrons per unit area).
Neutron fluence is important as neutrons straying from the reactor core cause damage to the materials that make up components of the reactor, such as the reactor pressure vessel (RPV). This neutron damage leads to a progressive undermining of the properties of the material, i.e. typically increased hardness, reduced fracture toughness, and reduced impact strength. FIG. 1 shows a graph of Charpy impact energy against temperature for un-irradiated and irradiated pressure vessel steel, and shows the reduced upper-shelf toughness and the shift in the ductile to brittle transition temperature (ΔDBTT) at an impact energy of 41 Joules (ΔTT41).
This “irradiation shift” must be understood if the behaviour of the materials comprising the nuclear reactor are to be effectively predicted. Predictive models are used to determine the irradiation shift behaviour and calculate remnant service life of the reactor. Critical to these models is a good understanding of the neutron fluence experienced by the reactor materials.
Fluence is measured using dosimetery. Conventional dosimetery is based on activation monitors. These are capsules containing foils of different metals which undergo an activation/reaction when neutrons of different energies impinge on them. FIG. 2 (taken from Pells, Fudge, Murphy and Watt, Use of Sapphire as a Neutron Damage Monitor for pressure vessel steels, ASTM STP 1001, p. 659-669, 1989) shows a schematic exploded view of such a monitor.
The dosimetery capsule is loaded into the reactor and used to measure the neutron fluence experienced by certain reactor components (e.g. the RPV). The reactions given in the table of FIG. 3 (taken from the Pells, Fudge, Murphy and Watt reference) indicate the reaction that different metals undergo and which neutron energies they represent.
There are, however, disadvantages with conventional dosimetery capsules. Firstly, the capsule is sealed within the reactor and can only be analysed and interrogated when the reactor is shut down, de-pressurised and opened for maintenance or re-fuelling. This means that it may be 10 to 20 years after reactor commissioning before the first measurement of neutron fluence experienced by critical components such as the RPV can be obtained. In the total life of a nuclear reactor it may only be possible to extract dosimetery capsules 2 or 3 times. This leads to a sparse database of neutron fluence information on which to base materials irradiation shift predictions. As a consequence, critical materials lifing decisions may be made with incomplete data and overly conservative materials models.
Secondly, as the table of FIG. 3 indicates, conventional dosimetery involves the activation of the metal foils. For example, Co60 is produced artificially by neutron activation of elemental foil of Co59 in the dosimetery capsule. Co60 then decays by negative beta decay to the stable isotope nickel-60 (Ni60), the activated Ni-atom emitting two gamma rays with energies of 1.17 and 1.33 MeV as indicated by the decay scheme shown in FIG. 4 (taken from http://en.wikipedia.org/wiki/Cobalt—60). However, the half-life of Co60 is 5.27 years, meaning that it remains radioactive during the period in which it must be measured and studied. Indeed, all dosimetery foils become radioactive in order to be measured. This makes it necessary to take many precautions while handling and studying dosimetery foils. For example, measurements must be performed remotely, in hot-cell facilities, which significantly increase the cost and complexity of the dosimetery interpretation process. An additional complication is that the half-lives of the dosimetery foils are different, such that the radiation record changes with time and this must be accounted for when calculating neutron fluence.
The sparse and discontinuous neutron fluence data available from dosimetery capsules make it necessary to use a complex process to calculate fluence. This process incorporates dosimetery data, physics-based predictions, and spectrum unfolding algorithms, as well as materials testing, reactor data, historical databases and past experience.
Thus, it would be desirable to provide a neutron fluence measuring device which avoids or overcomes at least some of the problems associated with dosimetery capsules.